Historical reliance on petroleum and other fossil fuels has been associated with dramatic and alarming increases in atmospheric levels of greenhouse gases. International efforts are underway to mitigate greenhouse gas accumulation, supported by formal policy directives in many countries. One central focus of these mitigation efforts has been development of processes and technologies for utilization of renewable plant biomass to replace petroleum as a source of precursors for fuels and other chemical products. The annual growth of plant-derived biomass on earth is estimated to approximate 1×10^11 metric tons per year dry weight. See Lieth and Whittaker (1975). Biomass utilization is, thus, an ultimate goal in development of sustainable economy.
Fuel ethanol produced from sugar and starch based plant materials, such as sugarcane, root and grain crops, is already in wide use, with global production currently topping 73 billion liters per year. Ethanol has always been considered an acceptable alternative to fossil fuels, being readily usable as an additive in fuel blends or even directly as fuel for personal automobiles. However, use of ethanol produced by these “first generation” bioethanol technologies does not actually achieve dramatic reduction in greenhouse gas emission. The net savings is only about 13% compared with petroleum, when the total fossil fuel inputs to the final ethanol outputs are all accounted. See Farrell et al. (2006). Moreover, both economic and moral objections have been raised to the “first generation” bioethanol market. This effectively places demand for crops as human food into direct competition with demand for personal automobiles. And indeed, fuel ethanol demand has been associated with increased grain prices that have proved troublesome for poor, grain-importing countries.
Great interest has arisen in developing biomass conversion systems that do not consume food crops—so-called “second generation” biorefining, whereby bioethanol and other products can be produced from lignocellulosic biomass such as crop wastes (stalks, cobs, pits, stems, shells, husks, etc. . . . ), grasses, straws, wood chips, waste paper and the like. In “second generation” technology, fermentable 6-carbon (C6) sugars derived primarily from cellulose and fermentable 5-carbon (C5) sugars derived from hemicellulose are liberated from biomass polysaccharide polymer chains by enzymatic hydrolysis or, in some cases, by pure chemical hydrolysis. The fermentable sugars obtained from biomass conversion in a “second generation” biorefinery can be used to produce fuel ethanol or, alternatively, other fuels such as butanol, or lactic acid monomers for use in synthesis of bioplastics, or many other products.
The total yield of both C5 and C6 sugars is a central consideration in commercialization of lignocellulosic biomass processing. In the case of ethanol production, and also production of lactate or other chemicals, it can be advantageous to combine both C5 and C6 sugar process streams into one sugar solution. Modified fermentive organisms are now available which can efficiently consume both C5 and C6 sugars in ethanol production. See e.g. Madhavan et al. (2012); Dumon et al. (2012); Hu et al. (2011); Kuhad et al. (2011); Ghosh et al. (2011); Kurian et al. (2010); Jojima et al. (2010); Sanchez et al. (2010); Bettiga et al. (2009); Matsushika et al. (2009).
Because of limitations of its physical structure, lignocellulosic biomass cannot be effectively converted to fermentable sugars by enzymatic hydrolysis without some pretreatment process. A wide variety of different pretreatment schemes have been reported, each offering different advantages and disadvantages. For review see Agbor et al. (2011); Girio et al. (2010); Alvira et al. (2010); Taherzadeh and Karimi (2008). From an environmental and “renewability” perspective, hydrothermal pretreatments are especially attractive. These utilize pressurized steam/liquid hot water at temperatures on the order of 160-230° C. to gently melt hydrophobic lignin that is intricately associated with cellulose strands, to solubilize a major component of hemicellulose, rich in C5 sugars, and to disrupt cellulose strands so as to improve accessibility to productive enzyme binding. Hydrothermal pretreatments can be conveniently integrated with existing coal- and biomass-fired electrical power generation plants to efficiently utilize turbine steam and “excess” power production capacity.
In the case of hydrothermal processes, it is well known in the art, and has been widely discussed, that pretreatment must be optimized between conflicting purposes. On the one hand, pretreatment should ideally preserve hemicellulose sugar content, so as to maximize the ultimate yield of monomeric hemicellulose-derived sugars. Yet at the same time, pretreatment should sufficiently expose and pre-condition cellulose chains to susceptibility of enzymatic hydrolysis such that reasonable yields of monomeric cellulose-derived sugars can be obtained with minimal enzyme consumption. Enzyme consumption is also a central consideration in commercialization of biomass processing, which teeters on the verge of “economic profitability” in the context of “global market economies” as these are currently defined. Notwithstanding dramatic improvements in recent years, the high cost of commercially available enzyme preparations remains one of the highest operating costs in biomass conversion.
As hydrothermal pretreatment temperatures and reactor residence times are increased, a greater proportion of C5 sugars derived from hemicellulose is irretrievably lost due to chemical transformation to other substances, including furfural and products of condensation reactions. Yet higher temperatures and residence times are required in order to properly condition cellulose fibers for efficient enzymatic hydrolysis to monomeric 6-carbon glucose.
In the prior art, an often used parameter of hydrothermal pretreatment “severity” is “Ro,” which is typically referred to as a log value. Ro reflects a composite measure of pretreatment temperature and reactor residence time according to the equation: Ro=tEXP[T−100/14.75] where t is residence time in minutes and T is reaction temperature in degrees centigrade.
Optimization of pretreatment conditions for any given biomass feedstock inherently requires some compromise between demands for high monomeric C5 sugar yields from hemicellulose (low severity) and the demands for high monomeric C6 sugar yields from cellulose (high severity).
A variety of different hydrothermal pretreatment strategies have been reported for maximizing sugar yields from both hemicellulose and cellulose and for minimizing xylo-oligomer inhibition of cellulase catalysis. In some cases, exogenous acids or bases are added in order to catalyse hemicellulose degradation (acid; pH<3.5) or lignin solubilisation (base; pH>9.0). In other cases, hydrothermal pretreatment is conducted using only very mild acetic acid derived from lignocellulose itself (pH 3.5-9.0). Hydrothermal pretreatments under these mild pH conditions have been termed “autohydrolysis” processes, because acetic acid liberated from hemicellulose esters itself further catalyses hemicellulose hydrolysis.
Acid catalysed hydrothermal pretreatments, known as “dilute acid” or “acid impregnation” treatments, typically provide high yields of C5 sugars, since comparable hemicellulose solubilisation can occur at lower temperatures in the presence of acid catalyst. Total C5 sugar yields after dilute acid pretreatment followed by enzymatic hydrolysis are typically on the order of 75% or more of what could theoretically be liberated from any given biomass feedstock. See e.g. Baboukaniu et al. (2012); Won et al. (2012); Lu et al. (2009); Jeong et al. (2010); Lee et al. (2008); Sassner et al. (2008); Thomsen et al. (2006); Chung et al. (2005).
Autohydrolysis hydrothermal pretreatments, in contrast, typically provide much lower yields of C5 sugars, since higher temperature pretreatment is required in the absence of acid catalyst. With the exception of autohydrolysis pretreatment conducted at commercially unrealistic low dry matter content, autohydrolysis treatments typically provide C5 sugar yields <40% theoretical recovery. See e.g. Diaz et al. (2010); Dogaris et al. (2009). C5 yields from autohydrolysis as high as 53% have been reported in cases where commercially unrealistic reactions times and extreme high enzyme doses were used. But even these very high C5 yields remain well beneath levels routinely obtained using dilute acid pretreatment. See e.g. Lee et al. (2009); Ohgren et al. (2007).
As a consequence of lower C5 yields obtained with autohydrolysis, most reports concerning hydrothermal pretreatment in commercial biomass conversion systems have focused on dilute acid processes. Hemicellulose-derived C5 sugar yields on the order of 85% have been achieved through use of so-called “two-stage” dilute acid pretreatments. In two-stage pretreatments, a lower initial temperature is used to solubilize hemicellulose, whereafter the C5-rich liquid fraction is separated. In the second stage, a higher temperature is used to condition cellulose chains. See e.g. Mesa et al. (2011); Kim et al. (2011); Chen et al. (2010); Jin et al. (2010); Monavari et al. (2009); Soderstrom et al. (2005); Soderstrom et al. (2004); Soderstrom et al. (2003); Kim et al. (2001); Lee et al. (1997); Paptheofanous et al. (1995). One elaborate “two-stage” dilute acid pretreatment system reported by the US National Renewable Energy Laboratory (NREL) claims to have achieved C5 yields on the order of 90% using corn stover as feedstock. See Humbird et al. (2011).
Notwithstanding the lower C5 yields which it provides, autohydrolysis continues to offer competitive advantages over dilute acid pretreatments on commercial scale.
Most notable amongst the advantages of autohydrolysis processes is that the residual, unhydrolysed lignin has greatly enhanced market value compared with lignin recovered from dilute acid processes. First, the sulphuric acid typically used in dilute acid pretreatment imparts a residual sulphur content. This renders the resulting lignin unattractive to commercial power plants which would otherwise be inclined to consume sulphur-free lignin fuel pellets obtained from autohydrolysis as a “green” alternative to coal. Second, the sulfonation of lignin which occurs during sulphuric acid-catalysed hydrothermal pretreatments renders it comparatively hydrophilic, thereby increasing its mechanical water holding capacity. This hydrophilicity both increases the cost of drying the lignin for commercial use and also renders it poorly suited for outdoor storage, given its propensity to absorb moisture. So-called “techno-economic models” of NREL's process for lignocellulosic biomass conversion, with dilute acid pretreatment, do not even account for lignin as a saleable commodity—only as an internal source of fuel for process steam. See Humbird et al. (2011). In contrast, the “economic profitability” of process schemes that rely on autohydrolysis include a significant contribution from robust sale of clean, dry lignin pellets. This is especially significant in that typical soft lignocellulosic biomass feedstocks comprise a large proportion of lignin, between 10 and 40% of dry matter content. Thus, even where process sugar yields from autohydrolysis systems can be diminished relative to dilute acid systems, overall “profitability” can remain equivalent or even better.
Autohydrolysis processes also avoid other well known disadvantages of dilute acid. The requirement for sulphuric acid diverges from a philosophical orientation favouring “green” processing, introduces a substantial operating cost for the acid as process input, and creates a need for elaborate waste water treatment systems and also for expensive anti-corrosive equipment.
Autohydrolysis is also advantageously scalable to modest processing scenarios. The dilute acid process described by NREL is so complex and elaborate that it cannot realistically be established on a smaller scale—only on a gigantic scale on the order of 100 tons of biomass feedstock per hour. Such a scale is only appropriate in hyper-centralized biomass processing scenarios. See Humbird et al. (2011). Hyper-centralized biomass processing of corn stover may well be appropriate in the USA, which has an abundance of genetically-engineered corn grown in chemically-enhanced hyper-production. But such a system is less relevant elsewhere in the world. Such a system is inappropriate for modest biomass processing scenarios, for example, on-site processing at sugar cane or palm oil or sorghum fields, or regional processing of wheat straw, which typically produces much less biomass per hectare than corn, even with genetic-engineering and chemical-enhancements.
Autohydrolysis systems, in contrast with dilute acid, are legitimately “green,” readily scalable, and unencumbered by requirements for elaborate waste water treatment systems. It is accordingly advantageous to provide improved autohydrolysis systems, even where these may not be obviously advantageous over dilute acid systems in terms of sugar yields alone.
The problem of poor C5 monomer yields with autohydrolysis has generally driven commercial providers of lignocellulosic biomass processing technology to pursue other approaches. Some “two-stage” pretreatment systems, designed to provide improved C5 yields, have been reported with autohydrolysis pretreatments. See WO2010/113129; US2010/0279361; WO 2009/108773; US2009/0308383; U.S. Pat. No. 8,057,639; US20130029406. In these “two stage” pretreatment schemes, some C5-rich liquid fraction is removed by solid/liquid separation after a lower temperature pretreatment, followed by a subsequent, higher temperature pretreatment of the solid fraction. Most of these published patent applications did not report actual experimental results. In its description of two-stage autohydrolytic pretreatment in WO2010/113129, Chemtex Italia reports a total of 26 experimental examples using wheat straw with an average C5 sugar recovery of 52%. These C5 recovery values do not distinguish between C5 recovery per se and monomer sugar yields, which is the substrate actually consumed in fermentation to ethanol and other useful products.
The introduction of a second pretreatment stage into a scheme for processing lignocellulosic biomass introduces additional complexities and costs. It is accordingly advantageous to substantially achieve the advantages of two-stage pretreatment using a simple single-stage autohydrolysis system.
We have discovered that, where single-stage autohydrolysis pretreatment is conducted to very low severity, it is possible to achieve unexpectedly high final C5 monomer yields of 55% theoretical yield and higher, while still achieving reasonable glucose yields. Where biomass feedstocks are pretreated to such low severity that the undissolved solids content of pretreated material retains a residual xylan content of at least 5.0% by weight, loss of C5 during pretreatment is minimized. Yet contrary to expectations, this very high residual xylan content can be enzymatically hydrolysed to monomer xylose, with high recovery, while sacrificing only a very small percentage of cellulose conversion to glucose, provided that sufficiently high xylanase and xylosidase activities are employed during enzymatic hydrolysis.
At these very low severity levels, the production of soluble by-products that affect cellulase activity or fermentive organisms is kept so low that the pretreated material can be used directly in enzymatic hydrolysis, and subsequent fermentation, typically without requirement for any washing or other de-toxification step.